Don't define away __attribute__ on gcc

Profile feedback sets -DNO_NORETURN, which causes the compat
header file to go into a default #else block. That #else
block defines away __attribute__(). Doing so causes all
kinds of problems with the Linux and gcc system headers:
in particular it makes the xmmintrin.h headers error out,
breaking the build.

Don't define away __attribute__ when __GNUC__ is set.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -291,10 +291,12 @@ extern char *gitbasename(char *);
 #else
 #define NORETURN
 #define NORETURN_PTR
+#ifndef __GNUC__
 #ifndef __attribute__
 #define __attribute__(x)
 #endif
 #endif
+#endif
 
 /* The sentinel attribute is valid from gcc version 4.0 */
 #if defined(__GNUC__) && (__GNUC__ >= 4)